1. Field of the Invention
The present invention relates to a display drive device for driving a display panel.
2. Description of the Related Art
In recent years, plasma display panels (PDPs) that have a thin thickness, a large screen, and high definition, have attracted attention. The PDP comprises a plurality of discharge cells (pixels) arranged in a matrix. An image is displayed on the PDP by utilizing light emission when discharge occurs in the discharge cells.
A typical AC PDP has a plurality of display electrodes arranged in parallel and a plurality of data electrodes arranged orthogonal to these display electrodes. A display drive device drives these data electrodes, and therefore, can be considered to drive capacitive load.
As the screen size, definition and luminance of PDPs have been more and more increased, display drive devices for driving the PDPs increasingly require a larger number of outputs and a higher voltage. Therefore, it is important to suppress power consumption and heat generation when the data electrode is driven.
When different potentials are applied to two data electrodes, a space between the electrodes functions as a capacitor. In other words, capacitive load is generated. When the capacitive load is driven, a large amount of power is consumed. As a conventional technique of reducing power consumption, the followings are known.
U.S. Pat. No. 7,116,137 discloses a display drive device that converts display data into a predetermined voltage level and outputs the resultant data to data electrodes. In the display drive device, a common floating potential line in a floating state is provided that is normally connected via selection switches to all output terminals in a wired OR manner. When a change in data level is detected on an output terminal at a timing of switching display data (i.e., between before and after display data is switched), the output terminal is controlled so that the output terminal temporarily goes to a high impedance (Hi-Z) at a predetermined timing (within a period when display data is switched and panel display is not performed). At the same time, the corresponding selection switch is controlled so that the output terminal is connected via the selection switch to the wired OR-connected floating potential line. By this control, display output is temporarily interrupted at output terminals at which data is changed by switching display data, so that all the output terminals are connected to the common floating potential line. In this case, the output terminals having a change in level are short-circuited, so that capacitance charges accumulated by displaying are transferred between terminals that have immediately previously output an H (=High) level and terminals that have immediately previously output an L (=Low) level. Therefore, the common floating potential line in the floating state is settled at a potential that is determined, depending on the number of H-level output terminals and the number of L-level output terminals. For example, when, of the terminals having a change in data, the number of H-output terminals is the same as the number of L-output terminals, the potential of the common floating potential line is ideally VDD/2 (VDD is the H-level potential of a display output). Therefore, the next drive operation only needs to be performed from VDD/2 to GND or VDD, whereby drive power can be reduced. Thus, this is a two-step drive technique.
U.S. Pat. No. 7,319,347 discloses another two-step drive technique in which panel electrodes and a capacitor having a load capacitance sufficiently larger than that accumulated between each panel electrode are connected to a common line corresponding to the floating potential line of U.S. Pat. No. 7,116,137 described above. The capacitance potential of the capacitor is previously set to be VDD/2. Charge and discharge are performed between the capacitor capacitance and an output terminal that has a change in data level between before and after display data is switched, so as to transfer accumulated charges so that the output terminal has the potential VDD/2, which is advantageous to the next drive operation.